METHOD OF CONTROLLING VEHICULAR DRIVELINES INCLUDING A CONTINUOUSLY VARIABLE TRANSMISSION

Abstract: A method of regulating a torque-controlled vehicular driveline including an engine, a continuously-variable-ratio transmission and a final drive, in which driver demand for a rise in engine speed is programmed to initiate a response in three successive stages, in each of which the engine and final drive speeds are differently correlated. The invention includes drivelines so controlled.

Description: Method of control l i ng vehi cul ar dri vel i nes i ncl udi ng a conti nuously vari abl e tran smi ssi on This invention relates to vehicular drivelines including a continuously- variable-ratio transmission, or CVT, the input of which is connected to an engine or other prime mover, and the rotary output of which will be referred to as the final drive. While the invention is applicable to boats, aircraft, railed and other vehicles in general, it applies particularly to drivelines for automobile vehicles, in which the final drive is connected to the driven wheel or wheels. And while the invention is also applicable to vehicular drivelines including other types of CVT, for instance those of belt-driven, Kopp, or Beier type, and in which the ratio-varying element may be directly and mechanically controlled to determine the instantaneous ratio, the invention applies particularly to CVTs of the toroidal-race rolling- traction type in which there is no such direct mechanical control upon the rollers. Instead the angular setting to which they settle, and thus the ratio that they transmit, is determined by a balance between the resultant torque to which they are subjected, and the operating force applied to the carriages in which they are mounted. Such toroidal-race CVTs are now known in the art as being of "torque-controlled" type, and recent examples are described for instance in Patents GB-B-2227287 and GB-B-0356102. It is also now well understood in the art to control such drivelines so that the engine performs at all times in conformity with an electronic "map" constructed to optimise efficiency in some form. Figure 1 of the accompanying drawings is a typical graph in which the y-axis represents engine torque T and the x-axis engine speed N, with the hyperbolae 1 indicating lines of constant power. Ordinates 2 and 3 indicate idling and maximum engine speeds respectively. Function 4 represents the maximum torque of which the engine is capable over its full range of working speeds. Function 5 represents the preferred and predetermined "control line", to which the engine is programmed to work, when under power demand, to achieve optimum performance according to a chosen criterion. That criterion may for instance be optimum emission characteristics, or least knock. With sophisticated control systems, an engine can be programmed to work to alternative control lines from which the driver can select. Typically, and as shown in Figure 1 , function 5 is chosen to achieve maximum power per unit of fuel consumed. Function 8 indicates the typical correlation of T and N when demand is withdrawn and the engine is over-running. The graph of Figure 2 illustrates the typical correlation of engine speed N (y-axis) and vehicle speed V (x-axis) in an automotive vehicle driveline where the engine is controlled as indicated in Figure 1, and where the driven wheels of the vehicle are the final drive. Lines 9 and 10 correspond to ordinates 2 and 3 of Figure 1, and the sloping linear function 11 (which passes through the origin 0) corresponds with first gear ratio in a typical fixed-ratio transmission and thus defines the effective mimmum values of N and V at which it is judged desirable for the driveline to work in practice. Ordinate 12 marks the maximum safe value of V. The effective "area" within which the driveline is capable of working is therefore bounded by lines 9, 11, 10 and 12. Assume that the vehicle has been travelling in steady state at a vehicle speed V ] and engine speed N j indicated by the intersection of the two lines at point 13. Assume now that while other variables remain unchanged, the driver wishes to call for greater road speed. He does so by depressing the accelerator pedal and so demanding greater engine speed, say N2- The typical response of a known driveline, programmed to operate the engine to a control line 5 as shown in Figure 1, has been to reach a final stable state of engine speed N2 and vehicle speed V2 (point 14) in two stages. Firstly by "kickdown", that is to say by causing engine speed to rise very quickly to N2 at point 15. Then secondly by "upshift" - that is to say, change of transmission ratio at constant engine speed - to move from point 15 to 14. While efficient, such a change can be disconcerting to drivers, especially those who are used to the response of typical manual and automatic transmissions containing a number of fixed ratios. With such known transmissions, the driver knows that rises in engine and road speeds go together, in any single ratio. The note of the engine is a reassuring guide to vehicle speed, and to the rate of change of that speed- However, when the performance of the vehicle of Figure 2 moves between points 13 and 14, the message to the driver is less clear. Between points 13 and 14 the engine note rises sharply, but vehicle speed V does not change. Between points 15 and 14 the vehicle speed changes, but the constant engine note suggests otherwise. At no time, during the transition from 13 to 14, do acceleration and engine note correlate in a way familiar to a conventional driver. The present invention aims to modify the mutual control of the engine and CVT so that although the engine can still operate to an optimum-efficiency control line like item 5 in Figure 1, during at least part of the acceleration following a pedal depression (and in reverse, the deceleration following a relaxation) the engine note and the acceleration are correlated in a manner more familiar to a driver used to conventional manuals and automatics. The invention is defined by the claims, the contents of which are to be read as included within the disclosure of this specification. In particular the invention includes methods and apparatus as described by way of example with reference to the following further figures of the accompanying drawings, in which: - Figure 3 is generally similar to Figure 2, but illustrates two modes of operating according to the invention, and Figure 4 is a further graph correlating engine speed N with accelerator pedal position P. Figure 2 included line 11, indicative of the ratio (e.g. 5 mph vehicle speed per 1000 rpm engine revolution) of a typical first forward gear in a conventional fixed-ratio automobile transmission. Because zero vehicle speed correlates with zero engine rpm in any such fixed ratio, line 11 passes through origin 0. Line 11 is repeated in Figure 3, as is line 16 indicative of the ratio (e.g. 10 mph/1000 rpm) both of a typical fixed ratio second gear, and also of a typical synchronous ratio - that is to say, the ratio where change takes place between first and second regimes - in a two regime CVT of the toroidal race or other type. Figure 3 also includes further lines 17 - 19, indicative of typical fixed ratios - e.g. 20. 30 and 40 mph per 1000 rpm respectively - of third, fourth and fifth forward gears in fixed-ratio transmissions. Assume that a vehicle, fitted with a driveline according to the invention, is travelling in steady state as indicated by point 20. That is to say, vehicle speed is Vj (as at 13 and 15 in Figure 2) and the engine is running at its minimum permitted speed (say 1500 rpm) as indicated by line 9. The driver now depresses the accelerator pedal, to a degree which the driveline is programmed to interpret as demanding a engine speed corresponding to N 2 (as at points 15 and 14 in Figure 2). According to one mode of operation of the present invention the driveline responds to the driver's demand in three stages, as follows. Firstly a rapid rise ("kickdown") in engine speed to a value (referred to in the claims as "engage speed"), predetermined and built into the control system as the lowest speed at which the engine should be running at the beginning of a period of substantial acceleration. In a typical engine as shown in Figures 2 and 3, capable of running between an idling speed of 1500 rpm. and a maximum speed of say 5000 rpm, this speed could for example be 2500 rpm. In Figure 3 the "engage speed" is the same speed N, as in Figure 2 and is represented by line 6, and at the end of the first of the three stages of response point 13 has therefore been reached. Function 22, drawn through point 13 and the origin 0, is now pre-chosen by the control system as an ideal, equivalent fixed-ratio for the CVT and engine of the driveline to follow in the second stage of the response, during which the driveline moves from point 13 on line 6 to point 23 when engine speed N2 is attained. During this second stage of the acceleration response the driveline is therefore programmed to respond as if in fixed ratio, so that the driver "feels" the acceleration of the vehicle just as he would if the driveline had the fixed ratio represented by function 22. Once the demanded engine speed N2 is reached at point 23, the third stage of the response begins. During this the engine speed remains at V2, so that the engine note does not change. Any further rise in vehicle speed to a final value (e.g. to point 24, value V2 as in Figure 2), at which the force exerted on the vehicle by the engine is matched by the forces resisting the vehicle's motion will be accommodated by ratio change within the CVT. A possible drawback of the embodiment of the invention shown in Figure 3 and so far described is that for the complete acceleration (between points 20 and 24) shown in thai figure the "fixed-ratio" second stage, between points 13 and 23, may be unduly prolonged, because of the relatively gentle slope of function 22. Conversely, if acceleration of vehicle speed had been demanded from velocity V3 (point 26) to V4 (point 29) at the left-hand side of the graph, by way of a "fixed-ratio" second stage 27, 28 coinciding with the "first ratio" line 11, that stage would have been relatively brief because of the steepness of the ratio line. In the alternative embodiment of the invention also shown in Figure 3, such extremes are avoided by programming the driveline so that during the second, "fixed ratio" stage of the response to a driver's demand for acceleration, vehicle and engine speeds rise together at a rate parallel to a chosen fixed ratio line - in this case, the line 16 representing the typical synchronous ratio of a two-regime toroidal-race CVT, or the second forward gear of a typical fixed-ratio transmission. Now, as shown in broken lines, the acceleration of the vehicle from speed V j at point 20, to speed V2 at point 24, is by way of points 13 and 31 instead of 13 and 23, so that the second stage of the response is briefer than before but the third stage longer. Conversely for the acceleration from the V3 to V the second and third stages are now 27 - 32 and 32 - 29 instead of 27 - 28 and 28 - 29, so that the second stage is longer than before and the third stage briefer. In practice the three-stage response to a demand for acceleration, of a driveline according to the invention, will probably be fractionally slower than the known two-stage response described with reference to Figure 2. However that possible small increase in total time will be offset by the driver being able to sense the response to his demand in a way that has not been customary hitherto in CVT drivelines. With relation to Figure 3, it should -be noted that while such parameters as "engage speed", "limit speed" and the various notional ratios have all be shown for convenience as straight-line functions, it is within the scope of the invention and obviously within the capabilities of programmed driveline control that non-straight-line functions could be substituted. In particular, the value of "engage speed" could easily vary over the total range of vehicle speed. Figure 4 interprets the operation of the invention, as already described with reference to Figure 3, in relation to accelerator pedal depression P (x-axis). As before, the y-axis represents demanded engine speed N. As the left-hand section of Figure 4 indicates. a typical driveline according to the invention may be programmed so that the three-stage reaction to a driver's demand for acceleration does not begin until the pedal is depressed at least 30%. Thereafter a "limit speed" 35, of any value between idling speed 9 and maximum engine speed 10 may be demanded. In Figures 2 and 3 N2 was the single example shown of such a "limit speed". As Figure 4 indicates, the "engage speed" value may for instance rise steadily (slope 36) to a range or value which it holds over the majority of the working range of the driveline - e.g. the value of 2500 rpm, line 6 in Figure 3. However, as shown at the right-hand side of Figure 4, the driveline may conveniently be controlled so as to raise the engage speed to a much higher value 37, so that the second, "fixed ratio" stage of the response starts on a much higher engine speed if the accelerator pedal is depressed almost fully, for instance in response to an emergency. In Figure 4 the hatched part 38 of the graph indicates the "area" over which the second stage of the response can take place, and the hatched part 39 the area over which the first "kickdown" stage takes place, during which engine speed rises rapidly, but there is no substantial change in vehicle speed. As already described, for each acceleration demand, the third stage of the response begins when engine speed reaches "limit speed".

Claims: CLAIMS 1. A method of controlling a vehicular driveline including an engine or other prime mover a continuously-variable-ratio transmission (CVT) and a final drive, in which over a working range of engine speed the engine is constrained to work to a performance "map" correlating each value of that speed with a particular value of engine torque, characterised in that driver demand for a rise in engine speed is programmed to initiate a response in three successive stages as follows :- Stage 1 : initial engine speed rises quickly to a predetermined value ("engage speed") without any substantial rise in final drive speed; Stage 2: both speeds then continue to rise and to be related by a predetermined function until engine speed reaches a value ("limit speed") commensurate with the driver's demand, and Stage 3 : limit speed is then maintained during any further change in final drive speed. 2. A method according to Claim 1 in which, for all values of initial engine speed and limit speed, the predetermined function can be represented by a substantially linear function passing through the origin of a graph on which engine and vehicle speed are plotted against each other. 3. A method according to Claim 1 in which the predetermined function can be represented on a graphical plot of engine against vehicle speed as a line of substantially constant slope, irrespective of the values of initial and limit speeds. 4. A method according to Claim 3 in which the slope of the function is substantially equal to that of the function of a typical intermediate gear in a comparable fixed-ratio transmission. 5. A method according to Claim 4 in which the intermediate gear approximates to second gear, for instance 10 mph vehicle speed per 1000 rpm engine speed. 6. A method according to Claim 3, for controlling a vehicular driveline in which the ratio range of the CVT is extended by providing two regimes of operation with a synchronous change between regimes, and in which the predetermined function of stage (ii) of the response has a slope equal to that of the ratio existing when synchronous change takes place. 7. A method according to Claim 1 in which engage speed is related to driven demand, a higher demand triggering a higher value of engage speed. 8. A vehicular driveline controlled by a method according to any of the preceding claims.

Inventors: GREENWOOD CHRISTOPHER JOHN, GREENWOOD, CHRISTOPHER, JOHN
Applicants: GREENWOOD CHRISTOPHER JOHN, TOROTRAK DEV LTD, GREENWOOD, CHRISTOPHER, JOHN, TOROTRAK DEVELOPMENT LIMITED
Application year: 1994
Main classification: F16H61
IPC codes: F16H9, F16H15, F16H61